In typical extrusion manufacturing systems employing thermoplastic materials, or compounds thereof, the components of the molding stock are preblended to a substantially homogeneous mix or consistency in a preliminary operation such as with a Banbury or rubber roll mill. The essentially homogeneous molding stock from the preblending is then fed into and continuously processed through an extrusion molding apparatus which plasticizes or fluxes the homogeneous molding stock and then molds the plasticized or fluxed homogeneous material to shape.
When plasticizing or fluxing thermoplastic materials in conventional extrusion apparatus, the mechanical forces and uneven temperature conditions induce a separation of the melting thermoplastic material from the remaining solid material whereupon the molten and solid phases segregate themselves as they advance through the apparatus. The molten phase tends to migrate to the upstream area of the extruder screw channel or section between the screw flights whereas the remaining solid phase tends to migrate to the downstream area or side. Accordingly, in order to achieve a substantially uniform consistency or degree of melting or fluxing throughout the mass of thermoplastic compound or stock advancing through the apparatus, it has been customary to extend the duration of the extruder plasticizing operation by employing an apparatus of protracted length, i.e., increased length over dimension (L/D) ratio, or utilize complex and costly devices such as dual screw mixing extruders, or intricate screw designs.
However, the use of an extruder of extensive length or prolonged periods of working or plastication within the apparatus greatly increase the likelihood of over heating and thermal degradation of the compound or heat sensitive ingredients thereof, or premature curing or scorching if a curing agent is present, because of the lack of precision and/or uniformity of temperature control in conventional extrusion apparatus and methods. The relatively low thermal conductivity of organic polymeric materials and typical fillers for molding compounds significantly contributes to the occurrence of uneven temperature condition and localized overheating within the molding stock undergoing extrusion. Moreover, because of the heat sink effect of the mass of metal constituting the extruder apparatus, temperature regulation within the extruder is slow in response.